Add Get The Return Type Of An Async Function as a TypeScript TIL

typescript/get-the-return-type-of-an-async-function.md

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+# Get The Return Type Of An Async Function
+
+When working with TypeScript-first libraries like Prisma, you'll be working
+with functions that have useful, but complex type signatures. The TypeScript
+built-in [Utility
+Types](https://www.typescriptlang.org/docs/handbook/utility-types.html)
+make a big difference when working with these.
+
+For instance, a function that makes a couple Prisma calls with join'ed data
+will have a large return type that you can't easily recreate with imported
+types from the Prisma client.
+
+Given a function like this, we can start out extracting its return type with
+the
+[`ReturnType`](https://www.typescriptlang.org/docs/handbook/utility-types.html#returntypetype)
+utility type, passing it the `typeof` the function as the generic.
+
+```typescript
+type FuncReturnType = ReturnType<typeof getPostsForUser>
+```
+
+A function like this is going to be async, so its return type will be wrapped
+in a promise. We can "unwrap" the promise with
+[`Awaited`](https://www.typescriptlang.org/docs/handbook/utility-types.html#awaitedtype).
+
+```typescript
+type FuncReturnType = ReturnType<typeof getPostsForUser>
+type ResolvedFuncReturnType = Awaited<FuncReturnType>
+```
+
+We are often querying for lists of things, so the result will be an array of
+the type we are interested in. We can extract the type of an array with
+`[number]`.
+
+```typescript
+type FuncReturnType = ReturnType<typeof getPostsForUser>
+type ResolvedFuncReturnType = Awaited<FuncReturnType>
+type PostType = ResolvedFuncReturnType[number]
+```
+
+Putting it all together into a single line looks like this:
+
+```typescript
+type Post = Awaited<ReturnType<typeof getPostsForUser>>[number]
+```
+
+[source](https://twitter.com/jbrancha/status/1603505456458207232)